Method and apparatus for minimizing inclusions in a glass making process

ABSTRACT

An apparatus for forming a glass sheet with reduced zircon inclusions in the glass sheet is disclosed. In one embodiment, the apparatus comprises heating elements distributed vertically between the weirs of a forming wedge and the root of the forming wedge, and wherein a thermal barrier is disposed between adjacent heating elements. A method of using the apparatus is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)of U.S. Provisional Application Ser. No. 60/930,765 filed on May 18,2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fusion processes for producing sheet glassand, in particular, to fusion processes which employ a crystallineceramic isopipe. Even more particularly, the invention relates tocontrolling the formation of crystalline defects in sheet glass producedby fusion processes employing ceramic containing isopipes. Thetechniques of the invention are particularly useful when fusionprocesses are employed to produce glass sheets for use as substrates inthe manufacture of liquid crystal displays, e.g., AMLCDs

2. Technical Background

The fusion process is one of the basic techniques used in the glassmaking art to produce sheet glass. Compared to other processes known inthe art, e.g., the float and slot draw processes, the fusion processproduces glass sheets whose surfaces have superior flatness andsmoothness. As a result, the fusion process has become of particularimportance in the production of the glass substrates used in themanufacture of liquid crystal displays (LCDs).

The fusion process, specifically, the overflow downdraw fusion process,is the subject of commonly assigned U.S. Pat. Nos. 3,338,696 and3,682,609, to Stuart M. Dockerty. As described therein, molten glass issupplied to a trough formed in a refractory body known as an “isopipe”.

In an exemplary fusion downdraw process as described in the Dockertypatent, once steady state operation has been achieved, molten glassoverflows the top of the trough on both sides so as to form two halfsheets of glass that flow downward and then inward along the outersurfaces of the isopipe. The two sheets meet at the bottom or root ofthe isopipe, where they fuse together into a single glass sheet. Thesingle sheet is then fed to drawing equipment which controls thethickness of the sheet by the rate at which the sheet is drawn away fromthe root. The drawing equipment is located sufficiently downstream ofthe root so that the single sheet has cooled before coming into contactwith the equipment.

The outer surfaces of the final glass sheet do not contact any part ofthe outside surface of the isopipe during any part of the process.Rather, these surfaces see only the ambient atmosphere. The innersurfaces of the two half sheets which form the final sheet do contactthe isopipe, but those inner surfaces fuse together at the root of theisopipe and are thus buried in the body of the final sheet. In this way,the superior properties of the outer surfaces of the final sheet areachieved.

An isopipe used in the fusion process is subjected to high temperaturesand substantial mechanical loads as molten glass flows into its troughand over its outer surfaces. To be able to withstand these demandingconditions, the isopipe is typically and preferably made from anisostatically pressed block of a refractory material (hence the name“iso-pipe”). In particular, the isopipe is preferably made from anisostatically pressed zircon refractory, i.e., a refractory composedprimarily of ZrO₂ and SiO₂. For example, the isopipe can be made of azircon refractory in which ZrO₂ and SiO₂ together comprise at least 95wt. % of the material, with the theoretical composition of the materialbeing ZrO₂.SiO₂ or, equivalently, ZrSiO₄.

A source of losses in the manufacture of sheet glass for use as LCDsubstrates is the presence of zircon crystal inclusions (referred toherein as “secondary zircon crystals” or “secondary zircon defects”) inthe glass as a result of the glass' passage into and over the zirconisopipe used in the manufacturing process. The problem of secondaryzircon crystals becomes more pronounced with devitrification-sensitiveglasses which need to be formed at higher temperatures.

Zircon which results in the zircon crystals which are found in thefinished glass sheets has its origin at the upper portions of the zirconisopipe. In particular, these defects ultimately arise as a result ofzirconia (i.e., ZrO₂ and/or Zr⁺⁴+2O⁻²) dissolving into the molten glassat the temperatures and viscosities that exist in the isopipe's troughand along the upper walls (weirs) on the outside of the isopipe. Thetemperature of the glass is higher and its viscosity is lower at theseportions of the isopipe as compared to the isopipe's lower portionssince as the glass travels down the isopipe, it cools and becomes moreviscous.

The solubility and diffusivity of zirconia in molten glass is a functionof the glass' temperature and viscosity (i.e., as the temperature of theglass decreases and the viscosity increases, less zirconia can be heldin solution and the rate of diffusion decreases). As the glass nears thebottom (root) of the isopipe, it may become supersaturated withzirconia. As a result, zircon crystals (i.e., secondary zircon crystals)nucleate and grow on the bottom portion (e.g. root) of the zirconisopipe. Eventually these crystals grow long enough to break off intothe glass flow and become defects at or near the fusion line of thesheet. Moreover, if the temperature of the glass at the isopipe root istoo low, devitrification of the glass may occur. Thus, it is desirableto increase the temperature of the isopipe near the isopipe root.Unfortunately, raising the temperature near the root of the isopipe hashad the unpleasant effect of also increasing the temperature of themolten glass within the isopipe trough, decreasing the viscosity of theglass and hence impacting the mass flow distribution of the glass. Thischange in mass flow distribution can be compensated by tilting theisopipe, but only within a narrow range of angles. Heating at the top ofthe isopipe occurs because the heating elements typically used to modifythe temperature of the glass flowing down the sides of the isopipe arecontained within a common plenum. As illustrated in FIG. 1, an isopipe10, as commonly used today, comprises a plurality of heating elements 12a-12 d and 14 a-14 d distributed upward from root 14 along both sides ofthe isopipe. Heating elements 12 a-12 d and 14 a-14 d are containedwithin the structure of enclosure 16 and more particularly within commonplenum 18. As a result, an increase in the temperature of thebottom-most heating element has a noticeable effect on the temperatureat the top of isopipe 10.

SUMMARY

In accordance with an embodiment of the invention, an apparatus forforming a glass sheet is disclosed comprising a forming wedge comprisingweirs at the top of the forming wedge and forming surfaces that convergeto a root at the bottom of the forming wedge, a plurality of heatingelements disposed between the root and the weirs, an enclosure disposedabout the forming wedge, the enclosure comprising an inner wallseparating the forming wedge and the heating elements, a thermal barriercomprising a thermal resistance rating (RSI) greater than about 0.0004K*m²/W disposed between a bottom-most heating element of the pluralityof heating elements and an adjacent heating element above thebottom-most heating element, where K is in degrees Kelvin and W is inwatts.

In another embodiment of the present invention, an apparatus for forminga glass sheet is described comprising a forming wedge comprising weirsat the top of the forming wedge and forming surfaces that converge to aroot at the bottom of the forming wedge, a plurality of temperaturemodifying elements disposed between the root and the weirs, an enclosuredisposed about the forming wedge, the enclosure comprising an inner wallseparating the forming wedge and the temperature modifying elements, athermal barrier comprising a thermal resistance rating (RSI) greaterthan about 0.0004 K*m²/W disposed between a bottom-most temperaturemodifying element of the plurality of temperature modifying elements andan adjacent temperature modifying element above the bottom-mosttemperature modifying element, where K is in degrees Kelvin and W is inwatts.

In still another embodiment, a method of making a glass sheet isdisclosed comprising flowing a molten glass over a forming bodycomprising converging forming surfaces, forming a vertical temperaturegradient between a temperature T₁ at the top of the forming body and atemperature T₂ at the bottom of the forming body, drawing the moltenglass from the bottom of the forming body to form a glass sheet, andwherein the temperature T₂ at the bottom of the forming body issubstantially decoupled from the temperature T₁ at the top of theforming body such that a change in T₂ does not cause a substantialchange in T₁.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate an exemplary embodiment of theinvention and, together with the description, serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional drawing illustrating a conventional isopipehoused in a conventional enclosure without insulating thermal barriersbetween heating elements.

FIG. 2 is a plot showing temperature coupling between upper and lowerlocations within the enclosure of FIG. 1.

FIG. 3 is a cross sectional view of an enclosure comprising insulatingthermal barriers in accordance with an embodiment of the presentinvention.

FIG. 4 is a side perspective view of a heating element (glow bar) havinga non-uniform cross sectional shape so that the ends of the glow barsproduce more heat than the center of the glow bar.

FIG. 5 is a cross sectional view of another enclosure comprisinginsulating thermal barriers in accordance with an embodiment of thepresent invention.

FIG. 6 is a cross sectional view of an enclosure comprising insulatingthermal barriers in accordance with still another embodiment of thepresent invention.

FIG. 7 is a cross sectional view of an enclosure comprising insulatingthermal barriers and actively heated inner shield walls in accordancewith an embodiment of the present invention.

FIG. 8 is a side view of a grid of discrete heating elements separatehorizontally and vertically by insulating thermal barriers in accordancewith an embodiment of the present invention.

FIG. 9 is a plot of temperature as a function of vertical distance abovethe root of an isopipe under four conditions: (1) no insulating thermalbarriers, (2) a structural layer only (3) an insulating thermal barrierin accordance with embodiments of the present invention and (4)insulating thermal barriers between adjacent heating elements accordingto certain embodiments of the present invention.

FIG. 10 is a plot showing the temperature across the width of theisopipe for the case of no insulating thermal barriers and the case ofinsulating thermal barriers between the bottom most heating elements andthe next vertically adjacent heating element the weirs and at the rootof the isopipe.

FIG. 11 is a plot showing a predicted decrease in the amount ofsecondary zircon in the presence of insulating thermal barriers betweenthe bottom most heating elements and the next vertically adjacentheating element when compared to a base case having no insulatingthermal barrier, plotted as a function of the concentration of ZrO₂dissolved in the glass minus the saturation concentration of ZrO₂ forthe glass, against the perpendicular distance into the glass from theglass-isopipe interface.

FIG. 12 is a chart showing the power supplied to the heaters of threezones within an exemplary prior art fusion draw machine during heat upof the isopipe, and a plot of the temperature differential between theupper and lower portions of the isopipe, as a function of time.

FIG. 13 is a chart showing the power supplied to the heaters of threezones within the exemplary prior art fusion draw machine of FIG. 12after having insulation removed from upper portions of the machine (thetop of the enclosure) during heat up of the isopipe, and a plot of thetemperature differential between the upper and lower portions of theisopipe, as a function of time.

FIG. 14 is a chart showing the power supplied to the heaters of threezones within another exemplary fusion draw machine during heat up of theisopipe, and a plot of the temperature differential between the upperand lower portions of the isopipe, as a function of time.

FIG. 15 is a chart showing the power supplied to the heaters of threezones within the exemplary fusion draw machine of FIG. 14 during heat upof the isopipe, and after an insulating thermal barrier was installedbetween the bottom-most isopipe heaters and adjacent heaters, and a plotof the temperature differential between the upper and lower portions ofthe isopipe, as a function of time. Insulation was also removed fromabove the isopipe.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

As previously described in reference to FIG. 1, prior art enclosuredesigns for fusion glass forming machines have incorporated heatingelements 12 a-12 d and 14 a-14 d in a common, interconnected plenum 18.Since heat rises, the upper portion of plenum 18 and enclosure 16 isheated with a contribution from all heating elements within the commonplenum by both radiation and convection. Heat energy which mightotherwise be directed toward the sides or converging forming surfaces ofisopipe 10 by the bottom-most heating elements 12 a, 14 a is insteadlost to the upper portions of isopipe 10 through common plenum 18.Hence, the upper portions of the isopipe, and the molten glassoverflowing the upper portions, receive more heat, while the lowerportions of the isopipe receive less heat. This offset in heating maylead to an unacceptably large temperature differential ΔT between theupper and lower portions of the isopipe, when in fact a small ΔT issought.

It is known that the temperature difference between the glass at the topof the isopipe and the glass at the bottom of the isopipe should beminimized to reduce the amount of isopipe material (typically acrystalline ceramic material, such as zircon) that is dissolved into themolten glass flowing over the surfaces of the isopipe. See for example,U.S. Patent Publication No. 2003/0121287, the contents of which areincorporated herein by reference.

Inclusions in the molten glass can occur when the amount of dissolvedisopipe material exceeds the saturation level for the given glassconditions. If, for example, the isopipe is formed from zircon, zirconcrystal growth at the bottom of the isopipe may be promoted. Thedissolved material comes out of solution and may be deposited ascrystals on the surface of the isopipe. If allowed to grow sufficientlylarge, these crystals may break off and become entrained in the glassflow. This is unacceptable from a quality perspective. The followingdescription assumes a zircon isopipe, but it can be appreciated that theinvention described herein in various embodiments is applicable toisopipes formed from other materials.

Since two key contributing factors for the dissolution of the isopipeare time and temperature, one way to eliminate the re-growth of theisopipe constituent material is to reduce the maximum temperature of theglass in or on the isopipe, particularly at the hotter upper portions.At the same time, the temperature of the flowing glass must bemaintained above the liquidus temperature of the glass at the lowerextremes of the isopipe—the root region—to prevent devitrification ofthe glass. Thus, it is desirable to increase the temperature of theglass at the bottom of the isopipe while simultaneously decreasing thetemperature of the glass flowing over the top of the isopipe: in otherwords, to reduce the temperature differential or gradient between thetop of the isopipe and the bottom of the isopipe.

Another reason for controlling the temperature differential between theweir and root is to facilitate startup of the fusion draw machine. Thatis, to heat the isopipe to its appropriate operating temperature, forexample, after a repair shutdown. The isopipe is, generally a monolithicblock of a refractory material, that if not heated substantiallyuniformly, may crack during heat up resulting from thermal stresses. Ina conventional fusion draw machine, an increase in the power supplied tothe bottom-most heating element(s) to raise the temperature of the rootregion also increases the temperature at the top of the isopipe,potentially increasing the temperature differential between the weirsand root. Thus, the temperature at the top of the isopipe becomes thecontrolling factor during isopipe heat up. By reducing the couplingbetween the temperature at the top of the isopipe from the temperatureat the bottom of the isopipe, the temperature differential between thetop and bottom of the isopipe, and stresses resulting from unevenheating, can be reduced.

Attempts to reduce the temperature differential between the weirs andthe root may be difficult, since increasing the power directed to thebottom-most heating elements to raise the temperature of the roottypically also increases the temperature at the weirs as previouslydescribed. Thus, the temperature at the isopipe root is effectivelycoupled to the temperature at the isopipe weirs. This can be seen moreclearly with the aid of FIG. 2, where FIG. 2 depicts temperature at twolocations proximate an exemplary isopipe as a function of absolute time.Curve 20 depicts the temperature at the ceiling of enclosure 16proximate the top of isopipe 10, whereas curve 22 depicts thetemperature proximate the root of the isopipe. The temperature scale atthe left of FIG. 2 corresponds to curve 20 while the scale at the rightof FIG. 2 corresponds to curve 22. In the present experiment, theelectrical power to the upper heating elements (e.g. 12 b-12 d and 14b-14 d in FIG. 1) was maintained constant over the time interval t₀ tot₁, while the power to the bottom-most heating elements (e.g. heatingelements 12 a, 14 a) was increased over the same time interval. Asexpected, the temperature at the lower portion of the isopipe, proximatethe root, increases during the interval t₀ to t₁; a total of about 7° C.over a period of approximately 3 hours. Also very clearly shown is thatthe temperature proximate the top of the isopipe also increased overthat period of time, about 4° C., demonstrating the temperature couplingbetween the two regions of the enclosure. The present invention isdirected toward uncoupling these upper and lower temperatures.Preferably, by practicing the present invention, an increase intemperature at the bottom of the isopipe of about 1° C. per hour resultsin an increase in temperature at the top of the isopipe of less thanabout 0.5° C. per hour.

In accordance with an embodiment of the present invention, and asillustrated in FIG. 3, an exemplary glass forming apparatus is shownwherein molten glass 24 is supplied to trough 26 defined by weirs 28 ofrefractory body 10 (isopipe 10). Molten glass 24 overflows the top ofweirs 28 on both sides of the isopipe to form two sheets of glass thatflow downward and then inward along the outer surfaces of the isopipe,including converging forming surfaces 30, 32 and vertical formingsurfaces 31, 33. The line of intersection between forming surfaces 30,31 and between 32, 33 is referred to as the “break” B. Convergingforming surfaces 30, 32 meet at root 14. The two sheets of molten glassflowing over the weirs and the forming surfaces meet at root 14, wherethey fuse together into single glass sheet 36. Glass sheet 36 is thenfed to drawing equipment (shown as pulling rolls 38 in FIG. 3), whichcontrols the thickness of the sheet by the rate at which the sheet isdrawn away from root 14. The drawing equipment is located sufficientlydownstream of the root so that the glass sheet has cooled sufficientlyto become substantially rigid before coming into contact with thedrawing equipment.

The glass forming process described supra is known as a fusion downdrawprocess. As can be seen from FIG. 3, the outer surfaces of the glassflowing over the sides of the isopipe do not contact any part of theoutside surface of the isopipe. On the other hand, the inner surfaces ofthe two half sheets which form the final glass sheet do contact theisopipe, but those inner surfaces fuse together at the root of theisopipe and are thus buried in the body of the final sheet. In this way,the superior properties of the outer surfaces of the final glass sheetobtainable with a fusion process are achieved.

Isopipe 10 is further disposed within enclosure 40. Enclosure 40substantially surrounds isopipe 10 and is used to maintain and controlthe temperature of the isopipe and the overflowing molten glass 24.Enclosure 40 comprises inner shield wall 42, and heating elements 44a-44 d and 46 a-46 d distributed at a vertical elevation above root 14but behind inner wall 42 such that inner wall 42 separates heatingelements 44 a-44 d and 46 a-46 d from isopipe 10. While four heatingelements are shown per side of the isopipe, there may be more than orless than four heating elements per side. Additional heating elementsmay be deployed below the root. Enclosure inner wall 42 is sometimesreferred to as muffle 42. Inner wall or muffle 42 preferably comprises atemperature resistant, but heat conductive, material and serves todiffuse the heat absorbed from heating elements 44 a-44 d and 46 a-46 d,and provide more even heating to isopipe 10. For example, SiC or hexaloyis a suitable material for inner wall 42. Heating elements 44 a-44 d and46 a-46 d may be, for example, electrically resistive heating elementsand may comprise metal bars (glow bars) extending horizontally along thewidth of isopipe 10 (from one longitudinal end of the isopipe to theother longitudinal end of the isopipe), and connected to a suitableelectrical supply.

Each of the plurality of heating elements 44 a-44 d and 46 a-46 d shownin FIG. 3 is preferably individually controlled using appropriatewell-known heating control methods and equipment. Heating elements 44a-44 d and 46 a-46 d may for example, be manually controlled,thermostatically controlled, or preferably computer controlled so thatan appropriate temperature profile may be more easily maintained withinenclosure 40.

Since the glass sheet drawn from the isopipe tends to have a hottertemperature at the center of the sheet that at the edges of the sheet, ahigh temperature at the ends of the heating elements serves to betterequalize the temperature across the width of the glass sheet. This canbe effective to reduce stress in the sheet and reduce stress inducedshape, or distortion (e.g. bowing) of the sheet. To that end, one ormore of heating elements 44 a-44 d and 46 a-46 d may have a crosssectional shape or profile which varies along the length of the heatingelement or elements such that the heat radiated from the ends of theheating elements is greater than the heat radiated from the center ofthe heating elements. For example, bottom-most heating elements 44 a, 46a may be adapted such that the resistance of the heating elements at theends of the elements are higher than the resistance of the heatingelements at the center of the elements. One way of attaining a varyingelectrical resistance is to vary the cross-sectional area of the heatingelement along the length of the heating element. FIG. 4 shows a heatingelement (generically 44, 46) having end portion 43 with a reduced crosssectional area when compared to a center portion 45 having a largercross sectional area. Thus, for a given electrical current traversingthe heating elements, the ends of the heating element will be heatedmore than the center of the heating elements, assuming a homogeneousmaterial. Alternatively, one or more of the heating elements maycomprise multiple electrically conductive materials with differentelectrical resistances.

Referring again to FIG. 3, an insulating barrier 48 is provided withinthe enclosure plenum 49 to thermally isolate the bottom-most heatingelements 44 a, 46 a from the remainder of the heating elements ofenclosure 40. That is, to thermally isolate heating elements 44 a and 46a from heating elements 44 b-44 d and 46 b-46 d, respectively, and aidin decoupling the temperature at the top portions of the isopipe fromthe bottom portions of the isopipe. As a result, the upper portions(e.g. weirs 28) of the isopipe become cooler because they receive lessheat from the bottom-most heating elements 44 a, 46 a, and the lowerportions of the isopipe (e.g. converging forming surfaces 30, 32 androot 34) become hotter because they receive more of the power from thebottom-most heating elements. The difference in temperature ΔT betweenthe upper portion of the isopipe (trough and weirs), and the lowerportions of the isopipe (e.g. converging forming surfaces and theisopipe root), and therefore the ΔT of the molten glass flowing overthem, is reduced. In general terms, for the upper portions of theisopipe (e,g, trough and weirs), the measured temperature of the glasswill be about the same as the temperature of the outer surface of theisopipe, while for the lower portions (e.g. converging forming surfacesand root), the temperature of the glass will typically be lower than thetemperature of the outer surface of the isopipe.

Insulating barrier 48 may be comprised of one or more layers. Forexample, one layer may be a layer that provides structural integrity orstrength to the barrier, while another layer or layers provides themajority of the insulating properties of the barrier. It has been foundthat a SiC barrier, for example, in and of itself is insufficient toprovide the needed thermal properties. Thus, in some embodiments,insulating thermal barrier 48 comprises a structural layer 48 a (e.g.SiC) and a thermally insulating layer 48 b. The structural layerpreferably supports the insulating layer. Additionally, since heatingelements occasionally fracture and fall, the structural layerbeneficially provides some mechanical protection to the bottom-mostheating elements, and components of the fusion draw machine belowinsulating thermal barrier 48. The insulating layer may comprise, forexample, a high temperature ceramic fiberboard, such as Duraboard® 2600.Preferably, insulating thermal barrier 48 has a thermal resistance valuerating (RSI) greater than about 0.0004 K*m²/W, more preferably greaterthan about 0.01 K*m²/W, and even more preferably greater than about 0.09K*m²/W, where K is degrees Kelvin and W is watts. By way of example andnot limitation, a Duraboard® 2600 thermal insulating layer having athickness of approximately 2.5 cm has been shown to have an RSI of atleast about 0.09 K*m²/W.

The reduced weir temperature derived from practicing the presentinvention results in reduced dissolution of isopipe constituentmaterial, e.g. zirconia, into the flowing molten glass, and an increasedtemperature at the isopipe root results in less precipitated material,e.g. zircon crystals, at the root of the isopipe. It should be notedthat although specific examples used herein are generally related tozircon-containing isopipes, the present invention is useful formitigating dissolution and precipitation of other isopipe materials thatmay be present as well. Preferably, the presence of precipitatedcontaminants/inclusions in the finished glass is limited to less thanabout 0.3 defects per pound of finished glass, more preferably less thanabout 0.1 defects per pound of finished glass, and even more preferablybelow about 0.09 defects per pound of finished glass. Additionally,decoupling the weir temperature from the root temperature can facilitatea reduction in weir temperature without decreasing the temperature ofthe root. For example, power to the upper heating elements may besubsequently reduced. Thus, the forming of glasses having smallerviscosity ranges between the viscosity of the glass delivered to theisopipe and the liquidus viscosity of the glass may be possible.Moreover, by maintaining a lower trough/weir temperature, while at thesame time enabling a higher root temperature, sag or creep of theisopipe material can be decreased, thereby prolonging the life of theisopipe.

Enclosure 40 may also include active cooling elements 50 such as pipingfor conveying a cooling fluid through the enclosure from a cooling fluidsource (not shown), such as a chill water source. The cooling elementsmay be in addition to the heating elements, or they may replace one ormore heating elements. As with the heating elements, cooling elements 50may be individually controlled.

In some embodiments, the inner wall itself may be segmented such thatone portion of the inner wall is separated from another portion of theinner wall by an insulating barrier, as shown in FIG. 5. FIG. 5illustrates apparatus 100 comprising enclosure 140, inner shield wall142 and isopipe 10 disposed within enclosure 140. Enclosure 140 alsoincludes insulating thermal barriers 148 disposed between heatingelements 144 a (and 146 a) and heating elements 144 b (and 146 b).Insulating thermal barrier 148 is extended so as to divide inner wall142 into a lower portion 142 a and an upper portion 142 b. As in theprevious embodiment, insulating thermal barrier 148 may comprisemultiple layers, including a structural layer and an insulating layer.Preferably, insulating thermal barrier 148 has a thermal resistancevalue rating (RSI) greater than about 0.0004 K*m²/W, more preferablygreater than about 0.01 K*m²/W, and even more preferably greater thanabout 0.09 K*m²/W.

Each of the plurality of heating elements 144 a-144 d and 146 a-146 dshown in FIG. 5 is preferably individually controlled using appropriatewell-known heating control methods and equipment. As previouslydescribed, one or more of heating elements 144 a-144 d and 146 a-146 dmay be adapted such that more heat is radiated from the ends of theheating elements (corresponding to the edges of the glass sheet), thanfrom the center portions of the heating elements (corresponding to thecentral or quality area of the glass sheet). Although not shown,apparatus 100 may employ active cooling as in the previous embodiment.

In another embodiment, shown in FIG. 6, an apparatus 200 is depictedcomprising enclosure 240, inner shield wall 242 and isopipe 10 disposedwithin enclosure 240. Enclosure 240 also includes a plurality ofinsulating thermal barriers 248 disposed between adjacent heatingelements. Each insulating thermal barrier 248 may comprise multiplelayers, including an insulating layer and, optionally, a structurallayer. Preferably, each insulating thermal barrier 248 has a thermalresistance value rating (RSI) greater than about 0.0004 K*m²/W, morepreferably greater than about 0.01 K*m²/W, and even more preferablygreater than about 0.09 K*m²/W.

Each of the plurality of heating elements 244 a-244 d and 246 a-246 dshown in FIG. 6 is preferably individually controlled using appropriatewell-known heating control methods and equipment. One or more of heatingelements 244 a-244 d and 246 a-246 d may be adapted such that more heatis radiated from the ends of the heating elements (corresponding to theedges of the glass sheet), than from the center portions of the heatingelements (corresponding to the central or quality area of the glasssheet). Although not shown, apparatus 200 may employ active cooling asin the previous embodiments.

In still another embodiment of the present invention depicted in FIG. 7,apparatus 300 is illustrated comprising enclosure 340, lower innershield wall 342 a, upper inner shield wall 342 b and with isopipe 10disposed within enclosure 340. Enclosure 340 also includes insulatingthermal barrier 348 disposed between heating elements 344 a (346 a) andheating elements 344 b (346 b). Insulating thermal barrier 348 maycomprise multiple layers, including an insulating layer and a structurallayer. Preferably, the insulating thermal barrier 348 has a thermalresistance value rating (RSI) greater than about 0.0004 K*m²/W, morepreferably greater than about 0.01 K*m²/W, and even more preferablygreater than about 0.09 K*m²/w.

Upper inner wall portion 342 b is a passive conductor of heat fromheating elements 344 a-344 d and 346 a-346 d, while lower inner wallportions 342 a are themselves active heating elements. Active lower wallportions 342 a may be directly heated by passing a current through thewall portions, or the active wall portions may have heating elementsimbedded within or attached to the wall portions. Both actively heatedwall portions 342 a and heating elements 344 a, 346 a may be used inconjunction with each other, or one or the other of the wall portions342 a or the heating elements 344 a, 346 a may be deactivated.

In yet another embodiment of the present invention, a plurality ofheating elements may be extended both vertically and horizontally acrossthe height and width of the isopipe as shown in FIG. 8, wherein eachsegmented heating element, generically indicated by reference numeral444, is thermally isolated from an adjacent heating element by aplurality of vertical and horizontal thermal shields 448, therebysetting up a grid of thermally isolated heating elements. Byindividually controlling each individual heating element 444, a morerefined temperature distribution can be implemented for isopipe 10, bothacross both the width and height of the isopipe. For example, differentsections of the isopipe may be heated to higher or lower temperaturesthan other sections of the isopipe. Use of segmented heating elementscan eliminate the need for elaborate methods of producing varied amountsof heat from different areas of a single heating element. Rather thanutilizing a heating element with a varying cross sectional profile, forexample, multiple heating elements may be employed across the desiredspace to achieve the same end.

FIG. 9 illustrates modeled glass surface temperature data as a functionof vertical distance above the root for an exemplary zircon isopipeutilizing methods and apparatus according to embodiments of the presentinvention. The data show four separate conditions (1) baseline data froma conventional isopipe, showing a temperature differential between theweirs and the root of the exemplary isopipe of 57° C., with a maximumtemperature at the break; (2) data from an isopipe comprising anon-insulating barrier; (3) an insulating thermal barrier disposedbetween the bottom-most heating element and the vertically adjacentheating element above the bottom-most element according to an embodimentof the present invention and; (4) an insulating thermal barrier betweenadjacent heating elements. The data depict a temperature decrease at theupper portions of the isopipe for both the non-insulating barrier (2),and the insulating thermal barrier (3, 4) when compared to a commonplenum (i.e. no barrier). However, data (2) for the non-insulatingbarrier indicate a decrease in temperature at the upper portion of theisopipe of only about 2° C.-3° C.; for the insulating thermal barrier(3) the decrease in temperature is an additional 7° C.-9° C. improvementover the non-insulating barrier. Finally, while showing improvement(i.e. lower temperatures at the upper portions of the isopipe), themultiple insulating thermal barriers (4) produced somewhat poorerresults when compared to the single insulating thermal barrier data (3).The data was modeled from the middle section of a zircon isopipe, with ahexaloy inner enclosure wall. The non-insulating barrier was a hexaloyplate approximately 10 mm thick. The insulating barrier was a layer ofDuraboard having a thickness of 2.54 cm combined with the hexaloybarrier above. The electrical power provided to the heating elements wasthe same for the four situations. Numerical temperature results for theexample above are contained in the following table. All temperatures arein ° C.

(1) (2) (3) (4) Weir Temp. 1232.3 1230.6 1223.9 1226.3 Max. Temp. 1237.21234.1 1225.4 1227.2 Root Temp. 1175.8 1177.2 1176.8 1176.8 Weir-Root ΔT56.5 53.4 47.1 49.5 Max-Root ΔT 61.4 56.9 48.6 50.4

In another example shown in FIG. 10, temperature across the width of theisopipe was modeled for two cases: a base case without an insulatingthermal barrier and; a second case with an insulating thermal barrier 48between the bottom-most heating elements and the next verticallyadjacent heating elements. The modeling examined the temperature at theglass input end of the isopipe, the midpoint of the isopipe and theisopipe end opposite the input end (the “compression” end). The resultsare shown in FIG. 10 for both the temperature at the break line and theroot temperatures. Curves 500 and 502 show the temperature across theisopipe for the base case and the second case, respectively, at thebreak line. Curves 504 and 506 show the temperature across the isopipefor the base case and the second case, respectively, at the root. Asillustrated by FIG. 10, the temperature at the break decreased acrossthe width of the isopipe with the addition of the insulating thermalshield. In contrast, a comparison between curves 504 and 506 shows anincrease in root temperature at the inlet end of the isopipe, while thetemperatures at the mid point and compression end were substantiallyunchanged. Thus, the overall temperature differential between the breaktemperature and the root temperature decreased. With a lower breaktemperature (e.g. lower upper isopipe temperature), less isopipematerial will dissolve. At the same time, by at least maintaining theroot temperature, if not slightly increasing the root temperature, thepotential for devitrification is reduced.

EXAMPLE 1

Shown in FIG. 11 are additional modeling data showing the predicteddifference for the formation of precipitated zircon for a base casewithout a barrier (curve 600) and the case with an insulating thermalbarrier between the bottom-most heating elements and the next verticallyadjacent heating element (curve 602). The glass flow was assumed to beabout 1500 pounds per hour. Isopipe temperatures at the weir, break androot for the base case, depicted by curve 600, were 1232° C., 1213° C.and 1153° C., respectively. The same temperatures with the insulatingthermal shield (weir, break and root), depicted by curve 602, were 1228°C., 1205° C. and 1159° C., respectively. Precipitated zircon crystals(“secondary” zircon) form on the isopipe as a result of thesupersaturation of ZrO₂ at the isopipe-glass interface as the glass meltflows over the isopipe. To determine the presence of supersaturation, itis necessary to know the ZrO₂ concentration profile in the glass meltand to compare that concentration to the actual concentration of ZrO₂ inthe glass. Consequently, the left scale (y-axis) of the plot shows theconcentration of ZrO₂ in the glass minus the saturation concentration ofZrO₂ in the glass. The horizontal or x-axis indicates the perpendiculardistance from the surface of the isopipe in a direction into the glassmelt. FIG. 11 shows that when an insulating thermal barrier is placedbetween the bottom-most heating element (e.g. element 44 a) and the nextvertically adjacent heating element (e.g. element 44 b), the distanceinto the glass that secondary zircon will form is decreased, thereforelimiting the growth of secondary zircon on the isopipe. That is, thedistance zircon crystallization can occur from the surface of theisopipe is reduced as the availability of ZrO₂ is reduced or eliminated,thereby restricting the amount of growth a zircon crystal may undergo.FIG. 11 also shows that the concentration of ZrO₂ in the glass melt inexcess of the saturation concentration is reduced with the use of theinsulating thermal barrier.

EXAMPLE 2

Shown in FIG. 12 is a chart plotting on the left hand axis powersupplied to heating elements in kW, on the right hand axis thetemperature differential between the weirs and the root, and along thebottom axis (x axis) time in hours for a conventional isopipe andenclosure. Four curves are shown: three curves representing the powersupplied to three heating zones; one heating zone below the root (the“transition” zone), a lower muffle heating zone proximate the root (i.e.the “bottom-most” heating element as used herein, e.g. 12 a), and anupper muffle heating zone (heaters above the bottom-most heatingelement, e.g. 12 b, c and d). These curves are respectively referencedas curves 700, 702 and 704. Fourth curve 706 represents the temperaturedifferential between the weirs and root of the isopipe.

Shown in FIG. 13 is a chart plotting on the left hand axis powersupplied to heating elements in kW, on the right hand axis thetemperature differential between the weirs and the root, and along thebottom axis (x axis) time in hours for an isopipe and enclosureaccording to an embodiment of the present invention. Four curves areshown: three curves representing the power supplied to three heatingzones; one heating zone below the root (the “transition” zone), a lowermuffle heating zone proximate the root (i.e. the “bottom-most” heatingelement as used herein, e.g. 44 a), and an upper muffle heating zone(heaters above the bottom-most heating element, e.g. 44 b, c and d).These curves are respectively referenced as curves 800, 802 and 804.Fourth curve 806 represents the temperature differential between theweirs and root of the isopipe. In the fusion draw machine of FIG. 13, alayer of insulation was removed overtop the enclosure (e.g. enclosure16).

A comparison between the fusion draw machines behind FIG. 12 and FIG.13, which illustrate a heat up of their respective isopipes, shows thatalthough the power balance between the various heating zones varied withthe removal of insulation at the top of the enclosure, the temperaturedifferential between the root and the weirs did not vary significantly.

Shown in FIG. 14 is a chart plotting on the left hand axis powersupplied to heating elements in kW, on the right hand axis thetemperature differential between the weirs and the root, and along thebottom axis (x axis) time in hours for another conventional isopipe andenclosure (e.g. enclosure 16). Four curves are shown: three curvesrepresenting the power supplied to three heating zones; one heating zonebelow the root (the “transition” zone), a lower muffle heating zoneproximate the root (i.e. the “bottom-most” heating element as usedherein, e.g. 12 a), and an upper muffle heating zone (heaters above thebottom-most heating element, e.g. 12 b, c and d). These curves arerespectively referenced as curves 900, 902 and 904. Fourth curve 906represents the temperature differential between the weirs and root ofthe isopipe. The arrow in FIG. 14 indicates the location in curve 906 ofa thermocouple failure during the experiment.

Shown in FIG. 15 is a chart plotting on the left hand axis powersupplied to heating elements in kW, on the right hand axis thetemperature differential between the weirs and the root, and along thebottom axis (x axis) time in hours for an isopipe and enclosure (e.g.enclosure 40) according to an embodiment of the present invention. Fourcurves are shown: three curves representing the power supplied to threeheating zones; one heating zone below the root (the “transition” zone),a lower muffle heating zone proximate the root (i.e. the “bottom-most”heating element as used herein, e.g. 44 a), and an upper muffle heatingzone (heaters above the bottom-most heating element, e.g. 44 b, c andd). These curves are respectively referenced as curves 1000, 1002 and1004. Fourth curve 1006 represents the temperature differential betweenthe weirs and root of the isopipe. In the fusion draw machine behindFIG. 13, an insulating thermal barrier according to an embodiment of thepresent invention was installed between the bottom most heating elements(e.g. heaters 44 a, 46 a) and the adjacent heating elements (44 b, 46b). The insulating thermal barrier comprises structural layer and aninsulating layer. The non-insulating structural barrier was a hexaloyplate approximately 10 mm thick. The insulating barrier was a layer ofDuraboard having a thickness of about 2.54 cm. Insulation was alsoremoved from the top of the enclosure as was done for the fusion drawmachine of FIG. 13. The insulating thermal barrier had an RSI ofapproximately 0.09 K*m2/W.

Both FIG. 14 and FIG. 15 show a heat up of their respective isopipes. Acomparison between the curves of FIGS. 14 and 15 show a 50% decrease inthe root to weir temperature differential with the additional of theinsulating thermal barrier. The removal of insulation was not judged tohave contributed significantly to this change, as indicated by the dataof FIGS. 12 and 13. The large reduction in the weir-root temperaturedifferential demonstrates the potential for greatly reduced thermalstresses during the heat up process, and a reduced risk of isopipefracture.

It should be emphasized that the above-described embodiments of thepresent invention, particularly any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiments of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

1. An apparatus for forming a glass sheet comprising: a forming wedgecomprising weirs at the top of the forming wedge and forming surfacesbelow the weirs that converge to a root at the bottom of the formingwedge; a plurality of heating elements disposed proximate to the formingwedge; an enclosure disposed about the forming wedge, the enclosurecomprising an inner wall separating the forming wedge from the pluralityof heating elements; an insulating thermal barrier disposed between aadjacent pair of heating elements, where K is in degrees Kelvin and W isin watts.
 2. The apparatus according to claim 1 wherein the RSI of theinsulating thermal barrier is greater than about 0.0004 K*m²/W.
 3. Theapparatus according to claim 1 wherein the insulating thermal barrier isdisposed vertically between an adjacent pair of heating elements.
 4. Theapparatus according to claim 1 wherein the insulating thermal barrier isdisposed horizontally between an adjacent pair of heating elements. 5.The apparatus according to claim 1 further comprising a cooling elementdisposed between the root and the weirs and separated from the formingwedge by the inner wall.
 6. The apparatus according to claim 1 furthercomprising a plurality of insulating thermal barriers disposed betweenan adjacent pair of heating elements.
 7. The apparatus according toclaim 1 wherein the inner wall comprises segments separated by theinsulating thermal barrier.
 8. The apparatus according to claim 9wherein a segment of the inner wall comprises a heating element.
 9. Theapparatus according to claim 1 wherein the insulating thermal barriercomprises a plurality of layers.
 10. The apparatus according to claim 11wherein the plurality of layers comprises a structural layer and aninsulating layer.
 11. An apparatus for forming a glass sheet comprising:a forming wedge comprising weirs at the top of the forming wedge andforming surfaces that converge to a root at the bottom of the formingwedge; a plurality of heating elements disposed at an elevation abovethe root; an enclosure disposed about the forming wedge, the enclosurecomprising an inner wall separating the forming wedge and the heatingelements; an insulating thermal barrier disposed between a bottom-mostheating element of the plurality of heating elements and a verticallyadjacent heating element.
 12. The apparatus according to claim 13wherein the RSI of the insulating thermal barrier is greater than about0.0004 K*m²/W.
 13. The apparatus according to claim 13 furthercomprising a plurality of insulating thermal barriers disposed betweenthe heating elements.
 14. The apparatus according to claim 13 whereinthe plurality of heating elements comprises a plurality of horizontallyaligned heating elements and a plurality of vertically aligned heatingelements, and wherein each of the horizontally and vertically alignedheating elements is separated from an adjacent heating element by aninsulating thermal barrier.
 15. A method for forming a glass sheetcomprising: flowing a molten glass over a forming body comprisingconverging forming surfaces; forming a vertical temperature gradientbetween a temperature T₁ at the top of the forming body and atemperature T₂ at the bottom of the forming body; drawing the moltenglass from the bottom of the forming body to form a glass sheet; andwherein the temperature T₂ at the bottom of the forming body issubstantially decoupled from the temperature T₁ at the top of theforming body such that a change in T₂ does not cause a substantialchange in T₁.
 16. The method according to claim 15 wherein the step offorming comprises heating the molten glass with a plurality of heatingelements and a heating element of the plurality of heating elements isseparated from an adjacent heating element by an insulating thermalbarrier comprising a thermal resistance rating (RSI) greater than about0.004 K*m²/W.
 17. The method according to claim 16 wherein theinsulating thermal barrier has an RSI greater than about 0.0004 K*m²/W.18. The method according to claim 15 wherein a 1° C./hour change in T₂results in less than a 0.5° C./hour change in T₁.
 19. The methodaccording to claim 15 wherein the glass sheet comprises less than about0.3 inclusions per pound of finished glass.
 20. The method according toclaim 15 wherein the glass sheet comprises less than about 0.09inclusions per pound of finished glass.